assume cs:codesg
codesg segment
           mov ax, 4c00H
           int 21h

    start: mov ax, 0
    s:     nop
           nop

           mov di, offset s
           mov si, offset s2
           mov ax, cs:[si]
           mov cs:[di], ax

    s0:    jmp short s
    s1:    mov ax, 0
           int 21h
           mov ax, 0

    s2:    jmp short s1
           nop
codesg ends
end start